This invention relates to a process of separating halogenated dioxins and furans from exhaust gases from combustion plants; particularly from refuse incinerating plants.
The exhaust gases of combustion plants are known to contain halogenated dibenzodioxins and halogenated dibenzofurans. Some of the compounds are extremely toxic and for this reason must almost entirely be removed from the combustion exhaust gases. For the sake of brevity the halogenated dibenzodioxins and the halogenated dibenzofurans are described hereinafter as dioxins and furans.
A large number of processes of separating dioxins and furans from combustion exhaust gases are already known.
For instance, Published German Application 3,919,124 proposes for separating polycyclic hydrocarbons which are deleterious to health from exhaust gases of refuse incinerating plants a process in which the exhaust gases which have been prepurified to remove dust, HCl, HF, SO.sub.x, NO.sub.x, and heavy metals are subjected to an absorption-filtration process in the temperature range of 70.degree. to 160.degree. C. to remove the remaining polycyclic hydrocarbons and heavy metals. In the process the prepurified exhaust gases are first mixed in a reactor with finely divided absorbents and are subsequently delivered to a filtering separator, in which an absorbent cake is formed on the filter cloth and is flown through by the exhaust gases. This results in a substantially complete separation of polycyclic hydrocarbons. In the known process, activated carbon, molecular sieves, sodium sulfide, and hydrate of lime are used, e.g., as absorbents.
In other known processes, carbonaceous adsorbents are used in fixed or slipping beds for a separation of organic pollutants.
Finally, European Patent Specification 253,563 discloses a process of separating the vaporous deleterious organic compounds from a dust-containing hot exhaust gas stream coming from a combustion plant. That process includes also the removal of acid components from the exhaust gas. In that process the exhaust gas stream at a temperature from 135.degree. to 400.degree. C. is fed to a spray absorber chamber (spray absorber), in which an aqueous basic liquid absorbent is atomized and the exhaust gas is cooled to a temperature of 180.degree. to 90.degree. C. while the water contained in the liquid absorbent is evaporated. A particulate material is formed in the absorber chamber and contains the products of the reactions between the basic absorbent and the acid components of the exhaust gas and also contains unreacted absorbent. The particulate material and any remaining dust are separated from the exhaust gas in a particle separator which follows the absorber chamber. In the process, activated carbon powder at a rate of 1 to 800 mg per sm.sup.3 (sm.sup.3 =standard cubic meter) of exhaust gas is blown into the exhaust gas stream at least at one location, which is disposed in the absorber chamber or between the absorber chamber and the particle separator. In any case the activated carbon powder is separated in the particle separator together with the particulate material. The particle separator consists of a bag filter or an electrostatic precipitator.
All known processes have the disadvantage that their separation efficiency for dioxins and furans is not sufficient. Besides, in the process known from European Patent Specification 253,563 the operation of an electrostatic precipitator at 90.degree. to 180.degree. C. in the presence of carbon dust involves a safety hazard because the carbon may be ignited in the electrostatic precipitator. Information is also available that electrostatic precipitators act as so-called "dioxin breeders", in which dioxins and furans are formed under the action of the electric field when the electrostatic precipitator is operated at elevated temperatures.